1. Field of the invention
The present invention relates in general to packaging of an IC (integrated circuit) chip and more particularly to an IC package of the kind in which external leads are connected to the surface of a ceramic base by means of sealing glass such as low temperature glass.
2. Description of the Prior Art
In an IC package of the kind in which external leads are fixed by using sealing glass such as low temperature glass, a single layer ceramic base is used. The ceramic base is formed by pressing and thereafter sintered. The ceramic base is applied with glass powder paste on which the external leads are placed. The glass powder paste is then heated and baked to the ceramic base so that the external leads are secured to the ceramic base.
After an IC chip is placed on the center of the ceramic base and the external leads are connected to the base by wire bonding, a ceramic cap applied with glass powder paste is joined with the ceramic base in such a manner as to hold the external leads therebetween. The glass powder paste applied on the cap side is then heated to melt so that the external leads are glass-sealed between the base and the cap. In the above manner, a low-cost IC package can be obtained.
In such a prior art IC package, the external leads are formed from Koval, alloy 42 or the like material having such a thermal expansion coefficient proximate to that of a general ceramic material consisting of alumina. Further, a material for sealing glass which is applied at least on the cap side is low temperature glass containing Pb(lead) so that the IC chip is not heated up to an abnormally high temperature at the time of glass-sealing.
However, since Koval or alloy used for forming the external leads contains Fe, Ni, etc. as major constituents, the inductance L of the external lead is 15 times higher than that of the Cu-type lead (non-magnetic body). On the other hand, in the case where electric current flows through the power source lead and earth lead, a counter electromotive force E=-L / di / dt is generated. Accordingly, when the external leads are formed from a material of high inductance, the variation of electric potential at the power source lead and earth lead at the time of high-speed operation becomes large, thus making it difficult to attain high-speed operation of the integrated circuit.
Further, in the case where all of the external leads are connected to the integrated circuit directly by means of bonding wires, it is necessitated that the power source leads and earth leads correspond in number to the power source terminals and earth terminals of the integrated circuit, resulting in the necessity of a number of external leads and therefore a large-sized IC package.
Further, the leaded glass is higher in dielectric constant than ceramic such as alumina ceramic. Accordingly, the longer portion of the external lead is enclosed by the sealing glass, the slower it becomes the signal propagation through the external lead, thus being disadvantageous for high-speed operation.